Extensive use has been made of forklift trucks to lift up such objects as loads, cargos, freights and the like for the purpose of their shipment and transportation from one place to another. Conventional forklift trucks customarily carry a vertically oriented mast assembly to which a carriage assembly is mounted for elevational movement therealong to raise the load up to a desired elevation. The carriage assembly is provided with a pair of spaced apart, generally parallel forks that overhang from the carriage assembly so as to take up the weight of the objects. The up/down movement of the carriage assembly may be redered effective by the combination of a lift jack and a lift chain associated therewith. Side shift cylinders may also be employed to have the carriage assembly move laterally, if appropriate.
U.S. Pat. No. 4,392,773 dated Jul. 12, 1983 to Johannson teaches a carriage assembly with shiftable forks so designed that all vertical forces are taken up by a lower bar to render the upper portion of the carriage assembly relatively light and small, thus allowing clear forward and downward vision for the vehicle operator. A primary feature of the '773 patent is that substantially only the horizontal forces are transferred to an upper bar of a side shifter, with the vertical forces to a lower bar of the side shifter.
U.S. Pat. No. 4,896,748 dated Jan. 30, 1990 to Mikkelsen et al. discloses a full free lift mast assembly that can significantly reduce bending of carriage lift jacks, fluid leakage, missequencing and like problems. The full free lift mast assembly has a first pair of spaced apart uprights, a second pair of spaced apart uprights mounted on the first pair of uprights and elevationally movable therealong and a carriage mounted on the second pair of uprights. A first mast lift jack is connected between the first and second pairs of spaced apart uprights. A connecting arrangement serves to couple cylinders of first and second carriage lift jacks and maintains them parallel in a preselected overlapping relationaship with each other.
In addition to the mast assemblies as referred to above, use is made nowadays of a three stage mast assembly which has the ability to lift objects up to a further elevated position and which carries a number of attachment actuators to operate attachments optionally built to the mast assembly for special tasks. The typical three stage mast assembly includes an outer mast attached to the front end of a vehicle body, an intermediate mast elevationally movably mounted to the outer mast and an inner mast mounted to the intermediate mast for elevational movement therealong. Each of the outer, intermediate and inner masts is provided with a pair of spaced-appart parallel uprights and a so-called "window" is provided between the uprights of the inner mast so that the operator may have a forward look therethrough. Elevationally slidably mounted to the inner mast is a carriage assembly which has a pair of forks for supporting objects to be lifted or moved.
First and second chain sheaves are attached to the top inboard surface of the uprights of the intermediate mast to support first and second chains therearound, while first and second hose sheaves are disposed at the inboard side of the first and the second chain sheaves to support first and second fluid hoses therearound. The first and second chains suspend from the chain sheaves and are anchored at their rear ends to the outer mast and at their front ends to the bottom of the inner mast. Similarly, the first and second fluid hoses suspend from the hose sheaves and anchored at their rear ends to the outer mast and at their front ends to the carriage assembly.
Since the hose sheaves and the fluid hoses lie within the window defined by the uprights of the inner mast, the size of the window would be reduced significantly, narrowing the front vision of the operator to a great extent. The operator's vision will be further narrowed in such an operative condition wherein the carriage assembly alone is moved upwards into the upper region of the window with the intermediate and inner mast kept retracted. It would goes without saying that the operator shall have difficulty in operating the forklift truck in the reduced window size and the narrowed front vision.